INTRODUCTIONWhen people think of sand filtration, they automatically relate to municipal watertreatment facilities. In general that’s the arena that this classical filter has ruled, butit most certainly has found applications in pure industrial settings, oftentimes forniche applications where suspended solids and organic matter persist in processwaters. Very common applications that I have seen in Eastern Europe have beenpart of cooling and process water treatment plants, particularly for large coolingtower applications in refineries and coke chemical plants, where biological growthproblems can adversely impact on heat exchange equipment. Also, in many formerSoviet block republics, municipal water treatment plants were almost always partof large industrial complexes, so that both the communities and plant watertreatment requirements were met by a single operation. This leads to a very distinctset of problems that we in the U.S. and parts of Western Europe don’t face,because of the separation of operations, and one which is way beyond the scope ofthis volume.Sand filtration is almost never applied as the primary treatment method. Most oftenit is a pretreatment or final stage, but sometimes intermediate stage of watertreatment, and is most often used along with other filtration technologies, carbonadsorption, sedimentation and clarification, disinfection, and biological methods.The term sand filtration is somewhat misleading and stems from older municipalwastewater treatment methods. While there is a class of filtration equipment thatrelies principally on sand as the filter media, it is more common to employ multiplemedia in filtration methods and equipment, with sand being the predominant media.In this regard, the terms sand filtration and granular media filtration are consideredinterchangeable in our discussions. The design, operation and maintenance of thesesystems are very straightforward, and indeed may be viewed as the least complexor simplest filtration practices that exist. It is a very old technology and much of WATER AND WASTEWATER TREA”MJ3h’T TECHNOLOGIESlook at this technology and then try some Questionsfor Thinking and Discussing.Remember to refer to the Glossary at the end of the book if you run across anyterms that are unfamiliar to you.WASTEWATER TREATMENT PLANT OPERATIONSBefore getting into the subject of sand filtration, we should first attempt to put thetechnologies of municipal wastewater treatment into some perspective. Wastewatertreatment plants can be divided into two major types: biological andphysicalkhemical. Biological plants are more commonly used to treat domestic orcombined domestic and industrial wastewater from a municipality. They usebasically the same processes that would occur naturally in the receiving water, butgive them a place to happen under controlled con&tions, so that the cleansingreactions are completed before the water is discharged into the environment.Physical/chemical plants are more often used to treat industrial wastewatersdirectly, because they often contain pollutants which cannot be removed efficientlyby microorganisms-- although industries that deal with biodegradable materials,such as food processing, dairies, breweries, and even paper, plastics andpetrochemicals, may use biological treatment. Biological plants generally use somephysical and chemical processes also.A physical process usually treats suspended, rather than dissolved pollutants. It maybe a passive process, such as simply allowing suspended pollutants to settle out orfloat to the top naturally-- depending on whether they are more or less dense thanwater. Or the process may be aided mechanically, such as by gently stirring thewater to cause more small particles to bump into each other and stick together,forming larger particles which will settle or rise faster-- a process known asflocculation. Chemical flocculants may also be added to produce larger particles.To aid flotation processes, dissolved air under pressure may be added to cause theformation of tiny bubbles which will attach to particles.Filtration through a medium such as sand as a final treatment stage can result in avery clear water. In contrast -- ultrafiltration, nanofiltration, and reverse osmosis(RO) are processes which force water through membranes and can remove colloidalmaterial (very fine, electrically charged particles, which will not settle) and evensome dissolved matter. Absorption (adsorption, techcally) on activated charcoalis a physical process which can remove dissolved chemicals. Air or steam strippingcan be used to remove pollutants that are gasses or low-boiling liquids from water,and the vapors which are removed in this way are also often passed through bedsof activated charcoal to prevent air pollution. These last processes are used mostlyin industrial treatment plants, though activated charcoal is common in municipalplants, as well, for odor control.Examples of chemical treatment processes, in an industrial environment, would be:1. converting a dissolved metal into a solid, settleable form by precipitationwith an alkaline material like sodium or calcium hydroxide. Dissolved irondownloaded from http://www.moitruongxanh.infoWHAT SAND F'XLTRATION IS ALL ABOUT 237or aluminum salts or organic coagulant aids like polyelectrolytes can beadded to help flocculate and settle (or float) the precipitated metal.converting highly toxic cyanides used in mining and metal finishingindustries into harmless carbon dioxide and nitrogen by oxidizing themwith chlorine.destroying organic chemicals by oxidizing them using ozone or hydrogenperoxide, either alone or in combination with catalysts (chemicals whichspeed up reactions) and/or ultraviolet light.2.3.In municipal treatment plants, chemical treatment-- in the form of aluminum or ironsalts-- is often used for removal of phosphorus by precipitation. Chlorine or ozone(or ultraviolet light) may be used for disinfection, that is, killing harmfulmicroorganisms before the final discharge of the wastewater. Sulfur dioxide orsulfite solutions can be used to neutralize (reduce) excess chlorine, which is toxicto aquatic life. Chemical coagulants are also used extensively in sludge treatmentto thicken the solids and promote the removal of water. A conventional treatmentplant is comprised of a series of individual unit processes, with the output (oreffluent) of one process becoming the input (influent) of the next process. The firststages will usually be made up of physical processes that take out easily removablepollutants. After this, the remaining pollutants are generally treated further bybiological or chemical processes. These may convert dissolved or colloidalimpurities into a solid or gaseous form, so that they can be removed physically, orconvert them into dissolved materials which remain in the water, but are notconsidered as undesirable as the original pollutants. The solids (residuals orsludges) which result from these processes form a side stream which also has to betreated for disposal.Common processes found at a municipal treatment plant include:Preliminary treatment to remove large or hard solids that might clog or damageother equipment. These might include grinders (comminuters), bar screens, and gritchannels. The first chops up rags and trash; the second simply catches largeobjects, which can be raked off; the third allows heavier materials, like sand andstones, to settle out, so that they will not cause abrasive wear on downstreamequipment. Grit channels also remove larger food particles (i. e., garbage).Primary settling basins, where the water flows slowly for up to a few hours, toallow organic suspended matter to settle out or float to the surface. Most of thismaterial has a density not much different from that of water, so it needs to be givenenough time to separate. Settling tanks can be rectangular or circular. In eithertype, the tank needs to be designed with some type of scrapers at the bottom tocollect the settled sludge and direct it to a pit from which it can be pumped forfurther treatment-- and skimmers at the surface, to collect the material that floatsto the top (which is given the rather inglorious name of "scum".) The diagrambelow shows the operation of a typical primary settling tankdownloaded from http://www.moitruongxanh.info238 WATER AND WASTEWATER TREATMENT TECHNOLOGIESSecondary treatment, usually biological, tries to remove the remaining dissolvedor colloidal organic matter. Generally, the biodegradation of the pollutants isallowed to take place in a location where plenty of air can be supplied to themicroorganisms. This promotes formation of the less offensive, oxidized products.Engineers try to design the capacity of the treatment units so that enough of theimpurities will be removed to prevent significant oxygen demand in the receivingwater after discharge.There are two major types of biological treatment processes: attached growth andsuspended growth.In an attached growth process, the microorganisms grow on a surface, such as rockor plastic. Examples include open trickling filters, where the water is distributedover rocks and trickles down to underdrains, with air being supplied through ventpipes; enclosed biotowers, which are similar, but more llkely to use shaped, plasticmedia instead of rocks; and so-called rotating biological contacters, or RBC's,which consist of large, partially submerged discs which rotate continuously, so thatthe microorganisms growing on the disc's surface are repeatedly being exposedalternately to the wastewater and to the air. The most common type of suspendedgrowth process is the so-called activated sludge system. This type of systemconsists of two parts, an aeration tank and a settling tank, or clarijier. The aerationtank contains a "sludge" which is what could be best described as a "mixedmicrobial culture", containing mostly bacteria, as well as protozoa, fungi, algae,etc. This sludge is constantly mixed and aerated either by compressed air bubblerslocated along the bottom, or by mechanical aerators on the surface. The wastewaterto be treated enters the tank and mixes with the culture, which uses the organiccompounds for growth-- producing more microorganisms-- and for respiration,which results mostly in the formation of carbon dioxide and water. The process canalso be set up to provide biological removal of the nutrients nitrogen andphosphorus. Refer to Figure 1 for a simplified process flow sheet.recycle t pump9.1Figure 1. Simplified process flow sheet of activated sludge process. downloaded from http://www.moitruongxanh.infoWHAT SAND FILTRATION IS ALL ABOUT 239After sufficient aeration time to reach the required level of treatment, the sludge iscarried by the flow into the settling tank, or clarifier, which is often of the circulardesign. (An important condition for the success of this process is the formation ofa type of culture which will flocculate naturally, producing a settling sludge and areasonably clear upper, or supernatant layer. If the sludge does not behave thisway, a lot of solids will be remain in the water leaving the clarifier, and the qualityof the effluent wastewater will be poor.) The sludge collected at the bottom of theclarifier is then recycled to the aeration tank to consume more organic material. Theterm "activated" sludge is used, because by the time the sludge is returned to theaeration tank, the microorganisms have been in an environment depleted of "food"for some time, and are in a "hungry", or activated condition, eager to get busybiodegrading some more wastes. Since the amount of microorganisms, or biomass,increases as a result of this process, some must be removed on a regular basis forfurther treatment and disposal, adding to the solids produced in primary treatment.The type of activated sludge system that has just been described is a continuousflow process. There is a variation in which the entire activated sludge process takeplace in a single tank, but at different times. Steps include filling, aerating, settling,drawing off supernatant, etc. A system ldce this, called a sequencing batch reactor,can provide more flexibility and control over the treatment, including nutrientremoval, and is amenable to computer control.)Nutrient removal refers to the treatment of the wastewater to take out nitrogen orphosphorus, which can cause nuisance growth of algae or weeds in the receivingwater. Nitrogen is found in domestic wastewater mostly in the form of ammoniaand organic nitrogen. These can be converted to nitrate nitrogen by bacteria, if theplant is designed to provide enough oxygen and a long enough "sludge age" todevelop these slow-growing types of organisms. The nitrate whch is produced maybe discharged; it is still usable as a plant nutrient, but it is much less toxic thanammonia. If more complete removal of nitrogen is required, a biological processcan be set up which reduces the nitrate to nitrogen gas (and some nitrous oxide).There are also physicalkhemical processeswhich can remove nitrogen, especiallyammonia; they are not as economical fordomestic wastewater, but might be suited foran industrial location where no other biologicalprocesses are in use. (These methods includealkaline air stripping, ion exchange, and"breakpoint" chlorination.)Phosphorous removal is most commonlydone by chemical precipitation with iron oraluminum compounds, such as ferric chlorideor alum (aluminum sulfate). The solids whichare produced can be settled along with othersludges, depending on where in the treatmenttrain the process takes place. "Lime", or~~ ~The chemical formula forlimestone is CaCO, andupon burning formscalcium oxide (CaO),which is known as burntlime. Calcium oxide, whenmixed with water, formscalcium hydroxide(Ca(OH)3. Calciumhydroxide is used to treatwater as a coagulation aidalong with aluminumsulfate. downloaded from http://www.moitruongxanh.info240 WATER AND WASTEWATER TREATMENT TECHNOLOGIEScalcium hydroxide, also works, but makes the water very alkaline, which has to becorrected, and produces more sludge. There is also a biological process forphosphorus removal, which depends on designing an activated sludge system insuch a way as to promote the development of certain types of bacteria which havethe ability to accumulate excess phosphorus within their cells. These methodsmady convert dissolved phosphorus into particulate form. For treatment plantswhich are required to discharge only very low concentrations of total phosphorus,it is common to have a sand filter as a final stage, to remove most of the suspendedsolids which may contain phosphorus.Disinfection, usually the final process before discharge, is the destruction ofharmful (pathogenic) microorganisms, i.e. disease-causing germs. The object is notto kill every living microorganism in the water-- which would be sterilization-- butto reduce the number of harmful ones to levels appropriate for the intended use ofthe receiving water.The most commonly used disinfectant is chlorine, which can be supplied in theform of a liquefied gas which has to be dissolved in water, or in the form of analkaline solution called sodium hypochlorite, which is the same compound asEither slow or rapidfiltration(depends on size ofplantholume ofwater considerations).Rapid-sand filters force water ,through a 0.45-lm layer of sand(d=0.4-1.2mm) and work faster,needing a smaller area. They needfrequent back-washing.Slo w-sand filters (d=O. 15-0.35mm)require a much larger area butreduce bacteriological and virallevels to a greater degree. The top 1inch must be periodically scrapedoff and the filter occasionally back-washed.common household chlorinebleach. Chlorine is quiteeffective against mostbacteria, but a rather highdose is needed to kill viruses,protozoa, and other forms ofpathogen. Chlorine hasseveral problems associatedwith its.use, among them 1)that it reacts with organicmatter to form toxic andcarcinogenic chlorinatedorganics, such as chloroform,2) chlorine is very toxic toaquatic organisms in thereceiving water-- the USEPArecommends no more than0.011 parts per million(mg/L) and 3) it is hazardousto store and handle.Hypochlorite is safer, but stillproduces problems 1 and 2.Problem 2 can be dealt with by adding sulfur dioxide (liquefied gas) or sodiumsulfite or bisulfite (solutions) to neutralize the chlorine. The products are nearlyharmless chloride and sulfate ions. This may also help somewhat with problem 1.A more powerful disinfectant is ozone, an unstable form of oxygen containing threeatoms per molecule, rather than the two found in the ordinary oxygen gas which downloaded from http://www.moitruongxanh.infoWHAT SAND FILTRATION IS ALL ABOUT 241makes up about 21 % of the atmosphere. Ozone is too unstable to store, and has tobe made as it is used. It is produced by passing an electrical discharge through air,which is then bubbled through the water. While chlorine can be dosed at a highenough concentration so that some of it remains in the water for a considerabletime, ozone is consumed very rapidly and leaves no residual. It may also producesome chemical byproducts, but probably not as harmful as those produced bychlorine. The other commonly used method of disinfection is ultraviolet light. Thewater is passed through banks of cylindrical, quartz-jacketed fluorescent bulbs.Anything which can absorb the light, such as fouling or scale formation on thebulbs' surfaces, or suspended matter in the water, can interfere with theeffectiveness of the disinfection. Some dissolved materials, such as iron and someorganic compounds, can also absorb some of the light. Ultraviolet disinfection isbecoming more popular because of the increasing complications associated with theuse of chlorine.Sludge from primary settling basins, called primary or "raw" sludge, is a noxious,smelly, gray-black, viscous liquid or semi-solid. It contains very highconcentrations of bacteria and other microorganisms, many of them pathogenic, aswell as large amounts of biodegradable organic material. Because of the highconcentrations, any dissolved oxygen will be consumed rapidly, and the odorousand toxic products of anaerobic biodegradation (putrefaction) will be produced. Thegreasy floatable skimmings from primary treatment are another portion of thisputrescible solid waste stream. Inaddition to the primary sludge,wastewater plants with secondarytreatment will produce a "secondarysludge", consisting largely ofmicroorganisms which have grownas a result of consuming the organicwastes. While not quite soobjectionable, due to thebiodegradation which has alreadytaken place, it is still very high inpathogens and contains muchmaterial which will decay andproduce odors if not treated further.Ultimately, the sludge must all bedisposed of. The way in which thisis done depends on the quality ofthe sludge, and determines how itneeds to be treated. The mostdesirable final fate for these solidswould be for beneficial use inagriculture, since the material hasorganic matter to act as a soilTHE FINAL TOUCHES TOWATERG Disinfection - water completelyfree of suspended sediment, istreated with a powerful oxidizingagent usually chlorine, chlorine andammonia (chloramine), or ozone. Aresidual disinfectant is le# in thewater to prevent reinfection.Chlorine can form harmfulbyproducts and has suspected linksto stomach cancer and miscarriages.GpH adjustment - so that treatedwater leaves the phnt in the desiredrange of 6.5 to 8.5pH units. downloaded from http://www.moitruongxanh.info242 WATER AND WASTEWATER TREATMENT TECHNOLOGIESconditioner, as well as a some fertilizer value. This requires the highest quality"biosolids", free of contamination with toxic metals or industrial organiccompounds, and low in pathogens. At a somewhat lower quality, it can be used forsimilar purposes on non-agricultural land and for land reclamation (e.g.. stripmines). Poorer quality sludge can be disposed of by landfilling or incineration.One commonly used method of sludge treatment, called digestion, is biological.Since the material is loaded with bacteria and organic matter; why not let thebacteria eat the biodegradable material? Digestion can be either aerobic oranaerobic. Aerobic digestion requires supplying oxygen to the sludge; it is similarto the activated sludge process, except no external "food" is provided. In anaerobicdigestion, the sludge is fed into an air-free vessel; the digestion produces a gaswhich is mostly a mixture of methane and carbon dioxide. The gas has a fuel value,and can be burned to provide heat to the digester tank and even to run electricgenerators. Some localities have compressed the gas and used it to power vehicles.Digestion can reduce the amount of organic matter by about 30 to 70 percent,greatly decrease the number of pathogens, and produce a liquid with an inoffensive,"earthy" odor. This makes the sludge safer to dispose of on land, since the odordoes not attract as many scavenging pests, such as flies, rodents, gulls, etc., whichspread pathogens from the disposal site to other areas-- and there are fewerpathogens to be spread.A liquid sludge, which might contain 3 to 6% dry weight of solids, can bedewatered to form a drier sludge cake of maybe 15 to 25 percent solids, which canbe hauled as a solid rather than having to be handled as a liquid. Equipment usedto dewater sludge includes centrifuges, vacuum filters, and belt presses or plate-and-frame presses. Chemical coagulants are commonly added to help form largeraggregates of solids and release the water. Further processes such as compostingand heat drying can produce a drier product with lower pathogen levels. Anotherapproach involves treatment with lime (calcium oxide), which kills pathogens dueADDITIONAL STEPSr3 Heavy metal removal: most treatmentplants do not have special stages formetals but rely on oxygenation,coagulation and ion exchange infiltersto remove them. Where metals persist,additional treatment would be needed.r3 Troublesome organics: Activatedcarbon filters are required wheresoluble organic constituents are presentbecause many will pass straight throughstandard plants, e.g. pesticides, phenols,MTBE and so forth.to its highly alkaline nature aswell as the heat that is generatedas it reacts with the water in thesludge; this also results in a drierproduct. A final disposal methodwhich eliminates all of thepathogens and greatly reduces thevolume of the sludge isincineration. This is notconsidered a beneficial use,however, and is becoming lesspopular due to public concernsover air emissions.Sludges from physical-chemicaltreatment of industrial wastestreams containing heavy metals downloaded from http://www.moitruongxanh.infoWHAT SAND FILTRATION IS ALL ABOUT 243and non-biodegradable toxic organic compounds often must be handled ashazardous wastes. Some of these will end up in hazardous waste landfills, or maybe chemically treated for detoxification-- or even for recovery of some componentsfor recycling. Recalcitrant organic compounds can be destroyed by carefullycontrolled high-temperature incineration, or by other innovative processes, such ashigh-temperature hydrogen reduction.GRANULAR MEDIA FILTRATIONGranular media filtration is used for treating aqueous waste streams. The filtermedia consists of a bed of granular particles (typically sand or sand with anthraciteor coal). The anthracite has adsorptive characteristics and hence can be beneficialin removing some biological and chemical contaminants in the wastewater. Thismaterial may also be substituted for activated charcoal.The bed is contained within a basin and is supported by an underdrain system whichallows the filtered liquid to be drawn off while retaining the filter media in place.As water containing suspended solids passes through the bed of filter medium, theparticles become trapped on top of, and within, the bed. The filtration rate isreduced at a constant pressure unless an increase in the amount of pressure isapplied to force the wastewater through the filter bed. In order to prevent pluggingof the upper surface and uppermost depth of the bed, the filter is backflushed athigh velocity to dislodge the filtered particles. The backwash water contains highconcentrations of solids and issent to further treatment stepswithin the watsewater treatmentplant.The filter application is typicallyapplied to handling streamscontaining less than 100 to 200mg/Liter suspended solids,depending on the requiredeffluent level. Increased-suspended solids loading reducesthe need for frequentbackwashing. The suspendedsolids concentration of thefiltered liquid depends on theparticle size distribution, buttypically, granular media filtersare capable of producing aOne of the reasons why it is important toremove suspended solids in water is thatthe particles can act as a source of foodand housing for bacteria. Not only doesthis make microbiological control muchharder but, high bacteria levels increasethe fouling of distribution lines andespecially heat transfer equipment thatreceive processed waters (for example, inone’s household hot water heater). Theremoval of suspended contaminantsenables chemical treatments to be attheir primary jobs of scale and corrosionprevention and microbial control.

One of the reasons why it is important toremove suspended solids in water is thatthe particles can act as a source of foodand housing for bacteria. Not only doesthis make microbiological control muchharder but, high bacteria levels increasethe fouling of distribution lines andespecially heat transfer equipment thatreceive processed waters (for example, inone’s household hot water heater). Theremoval of suspended contaminantsenables chemical treatments to be attheir primary jobs of scale and corrosionprevention and microbial control. downloaded from http://www.moitruongxanh.info244 WATER AND WASTEWATER TREATMENT TECHNOLOGIESfiltered liquid with a suspended solids concentration as low as 1 to as high as 10mg/Liter. Large flow variations will affect the effluent’s quality.Granular media filters are usually preceded by sedimentation in order to reduce thesuspended solids load on the filter. Granular media filtration can also be installedahead of biological or activated carbon treatment units to reduce the suspendedsolids load and in the case of activated carbon to minimize plugging of the carboncolumns. Granular media filtration is only marginally effective in treating colloidalsize particles in suspensions. Usually these particles can be made larger byflocculation although this will reduce run lengths. In cases where it is not possibleto flocculate such particles (as in the case of many oil/water emulsions), othertechniques such as ultrafiltration must be considered. Figure 2 illustrates a commonsand filter that most people are familiar with in swimming pool applications. Suchsystems rely on very fine sand media that can typically remove suspended particlesabout 0.5 pm in size. Filtration is an effective means of removing low levels ofsolids from wastes provided the solids concentration does not vary greatly and thefilter is backwashed at appropriate intervals during the filtration cycle. Theoperation can be easily integrated with other treatment steps, and further, is wellsuited to mobile treatment systems as well as on-site or fixed installations. In short,sand filtration technologies, although simple, are quite versatile in meetingtreatment challenges.Figure 2. A simple sand filtration unit. downloaded from http://www.moitruongxanh.infoWHAT SAND FILTRATION IS ALL ABOUT 245A typical multi-media sand filtration unit is shown in Figure 3. In this configurationa coarse layer of media is used to reduce the contaminant loading to the final layer.This allows multimedia filters to use finer media. Such units generally removesuspended solids down to about 15 pm, and they require large volumes of water toproperly remove contaminant that is trapped deep within the bed. Oftenmanufactures of these types of systems claim 90 % removal of 0.5 pm particles andlarger. This can be a misleading statement as quite often only about 5 % of the 0.5pm particles will be removed. Grouping the 0.5 pm particles with much largerparticles allows the claim to be met by removing a few large volume particles fromthe tower sump, even though the vast majority of fine particles remain to foul heatexchange equipment.A typical physical-chemical treatment system incorporates three "dual" medial (sandanthracite) filters connected in parallel in its treatment train. The major maintenanceconsideration with granular medial filtration is the handling of the backwash. Thebackwash will generally contain a high concentration of contaminants and requiresubsequent treatment.6IFigure 3. Mulimedia sand filter. downloaded from http://www.moitruongxanh.info246 WATER AND WASTEWATER TREATMENT TECHNOLOGIESIn this application, the operations of precipitation and flocculation play importantroles. Precipitation is a physiochemical process whereby some, or all, of asubstance in solution is transformed into a solid phase. It is based on alteration ofthe chemical equilibrium relationships affecting the solubility of inorganic species.Removal of metals as hydroxides and sulfides is the most common precipitationapplication in wastewater treatment. Lime or sodium sulfide is added to thewastewater in a rapid mixing tank along with flocculating agents. The wastewaterflows to a flocculation chamber in which adequate mixing and retention time isprovided for agglomeration of precipitate particles. Agglomerated particles are thenseparated from the liquid phase by settling in a sedimentation chamber, and/or byother physical processes such as filtration.Precipitation is often applied to the removal of most metals from wastewaterincluding zinc, cadmium, chromium, copper, fluoride, lead, manganese, andmercury. Also, certain anionic species can be removed by precipitation, such asphosphate, sulfate, and fluoride. Note that in some cases, organic compounds mayform organometallic complexes with metals, which could lnhibit precipitation.Cyanide and other ions in the wastewater may also complex with metals, makingtreatment by precipitation less efficient. A cutaway view of a rapid sand filter thatis most often used in a municipal treatment plant is illustrated in Figure 4. Thedesign features of this filter have been relied upon for more than 60 years inmunicipal applications.Figure 4. Cutaway view of a rapid sand filter. downloaded from http://www.moitruongxanh.infoWHAT SAND FILTRATION IS ALL ABOUT 247PollutantFecal coliformBiochemical OxygenTotal SuspendedDemand (BOD)Solids (TSS)LET’S TAKE A CLOSER LOOK AT SAND FILTERSPercent Pollutant PercentRemoval Removal76 Total Organic 48Carbon (TOC)70 Total Nitrogen (TN) 2170 Iron, Lead, Zinc 45A typical sand filter system consists of two or three chambers or basins. The firstis the sedimentation chamber, which removes floatables and heavy sediments. Thesecond is the filtration chamber, which removes additional pollutants by filteringthe runoff through a sand bed. The third is the discharge chamber. The treatedfiltrate normally is then discharged through an underdrain system either to a stormdrainage system or directly to surface waters. Sand filters are able to achieve highremoval efficiencies for sediment, biochemical oxygen demand (BOD), and fecalcoliform bacteria. Total metal removal, however, is moderate, and nutrient removalis often low. Figure 5 illustrates one type of configuration. Typically, sand filtersbegin to experience clogging problems within 3 to 5 years. Accumulated trash,paper, debris should be removed every six months or as needed. Correctivemaintenance of the filtration chamber includes removal and replacement of the toplayers of sand and gravel as they become clogged. Table 1 provides some typicalremoval efficiencies for specific pollutants.GraIed Cover Solid Coni&ale (Fabric WrappdOver Entire Grate Opening)Figure 5. Example of a sand filter configuration.Table 1. Typical removal efficiencies. downloaded from http://www.moitruongxanh.info248 WATER AND WASTEWATER TREATMENT TECHNOLOGIESPRECIPITATION, FLOCCULATION AND AGGLOMERATIONWe will be examining these subjects in a little more detail in the next chapter. Butfor now, we should cover some of the basics because of their importance to sandfiltration. The process offlocculation is applicable to aqueous waste streams whereparticles must be agglomerated intolarger more settleable particles prior tosedimentation or other types oftreatment. Highly viscous waste streamswill inhibit the settling of solids. Inaddition to being used to treat wastestreams, precipitation can also be used asan in situ process to treat aqueous wastesin surface impoundments. In an in-situapplication, lime and flocculants areadded directly to the lagoon, and mixing,flocculation, and sedimentation areallowed to occur within the lagoon.Precipitation and flocculation can beintegrated into more complex treatment systems. The performance and reliabilityof these processes depends greatly on the variability of the composition of the wastebeing treated. Chemical addition must be determined using laboratory tests andmust be adjusted with compositional changes of the waste being treated or poorperformance will result.Precipitation is nonselective in that compounds other than those targeted may beremoved. Both precipitation and flocculation are nondestructive and generate alarge volume of sludge which must be disposed of. Coagulation, flocculation,sedimentation, and filtration, are typically followed by chlorination in municipalwastewater treatment processes.Coagulation involves the addition of chemicals to alter the physical state ofdissolved and suspended solids. This facilitates their removal by sedimentation andfiltration. The most common primary coagulants are alum ferric sulfate and ferricchloride. Additional chemicals that may be added to enhance coagulation includeactivate silica, a complex silicate made from sodium silicate, and charged organicmolecules called polyelectrolytes, which include large-molecular-weightpolyacrylamides, dimethyl-diallylammonium chloride, polyamines, and starch.These chemicals ensure the aggregation of the suspended solids during the nexttreatment step-flocculation. Sometimes polyclectrolytes (usually polyacrylamides)are also added after flocculation and sedimentation as an aid to the filtration step.Coagulation may also remove dissolved organic and inorganic compounds. Thehydrolyzing metal salts may react with the organic matter to form a precipitate, orthey may form aluminum hydroxide or ferric hydroxide floc particles on which theorganic molecules adsorb. The organic substances are then removed bysedimentation and filtration, or filtration alone if direct filtration or inline filtrationis used. Adsorption and precipitation also removes inorganic substances.The process of sedimentation involves the separation from water, by gravitationalsettling of suspended particles that are heavier than water. The resulting effluent isthen subject to rapid filtration to separate out solids that are still suspended in thewater. Rapid filters typically consist of 24 to 36 inches of 0.5 to 1-mm diametersand and/or anthracite. Particles are removed as water is filtered through the mediaat rates of 1 to 6 gallons/minute/square foot. Rapid filtration is effective inremoving most particles that remain after sedimentation. The substances that areremoved by coagulation, sedimentation, and filtration accumulate in sludge whichmust be properly disposed of.Coagulation, flocculation, sedimentation, and filtration will remove manycontaminants. Perhaps most important is the reduction of turbidity. This treatmentyields water of good clarity and enhances disinfection efficiency. If particles are notremoved, they harbor bacteria and make fmal disinfection more difficult.

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